The present invention relates to a broadband switching network composed of a network of crosspoint switching circuits of the type disclosed in U.S. Pat. No. 4,346,381.
The switching circuits of this switching network are constructed according to ECL (emitter coupled logic) technology and are composed of differential amplifier stages whose emitter branches each include a negative feedback resistor and a transistor. The base of this transistor is charged with a control signal so as to switch through or open the crosspoint. These differential amplifiers must receive additional reference signals. Moreover, for many applications, the switching network of U.S. Pat. No. 4,346,381 provides insufficient decoupling between the signal lines in the row and column directions. In this switching network, it is not very easily possible to combine a plurality of crosspoints of a column into groups and switch them to one output stage.
U.S. Pat. No. 4,443,773 discloses a broadband switching network in matrix form in which compensation elements are provided to eliminate residual overcoupling at blocked crosspoints. However, this switching network requires ground-symmetrical amplifiers which brings about added expenditures for supply lines and switching elements.
Federal Republic of Germany Offenlegungsschrift (Laid-Open Application) No. 2,917,989 discloses a switching network which employs FET's (field effect transistors) as crosspoints. These crosspoints are of asymmetrical design. At higher frequencies, the reverse resistances of these FET's are known to be determined by the parasitic capacitances between their electrodes. To avoid this drawback, the switching network disclosed in that publication provides that the output signal of a column is returned, via an amplifier stage, to the bulk terminals of the FET's. The load capacitances, however, can be reduced by this measure only by about three times their value. It is not possible with this arrangement to link very many outputs together. Additionally the transmittable bit rate is considerably lower than that which can be achieved using bipolar technology.